Chromate production



Patented .Fune ii, 3

CHROMATE PRODUCTION Marvin J. Udy, Niagara Falls, N. Y.

No Drawing.

Application February 15, 1939,

Serial No. 256,559

Claims. 1

This invention relates to the production of chromium compounds and has for an object the provision of certain improvements in processes for producing hexavalent chromium compounds. More particularly, the invention contemplates the provision of certain improvements in processes for producing hexavalent chromium compounds from chromium alloys such as ferrochromium. A further object of the invention is to provide an improved process for producing calcium chromate.

In my United States Patent No. 1,926,758, dated September 12, 1933, I have disclosed a process for producing calcium chromate in which a mixture of finely divided ierrochromium and finely divided lime is subjected to an oxidizing treatment at temperatures in the range, 600 C. to 1000 C., a small amount of an oxidation promoter being employed also to promote conversion of the elemental chromium to the hexavalent condition. In practicing the process of my aforementioned patent, substantial conversion of chromium to the hexavalent condition, under the conditions recited in the patent requires treatment for from five to twenty hours.

In my later U. S. Patent No. 2,176,685 dated October 17, 1939, I have disclosed a process in which ferrochromium, in finely divided condition, is oxidized at temperatures below 1000 C. in the presence of lime and soda ash for the production of calcium chromate and sodium chromate.

The present invention is based on my discovery that higher degrees of conversion of chromium to the hexavalent condition can be accomplished with advantage by carrying out the oxidation treatment in two or more stages to convert the chromium initially to the trivalent condition without substantial conversion of the chromium to the hexavalent condition, and subsequently to eii'ect conversion of the trivalent chromium to the hexavalent condition.

The process of the invention may be carried out advantageously by subjecting a chromium alloy such as ferrochromium, in finely divided condition, to an oxidizing treatment at a high temperature below its fusing point at which hexavalent chromium will not form or exist to form a product substantially free of metallic iron and metallic chromium and in which the chromium is present largely or substantially entirely in the trivalent condition, and thereafter subletting the trivalent chromium-bearing product to an oxidizing treatment at a relatively low temperature at which hexavalent chromium will form and exist. The invention provides, in effect,

a two-stage operation for the ultimate production of hexavalent chromium. In the first stage, terrochromium (metallic iron and metallic chromium) is subjected to oxidation, and, in the second stage, a mixture of oxides of iron and chromium is subjected to oxidation, oxidation of the chromium being progressive-from a, zero valence to a valence of three in the first stage, and from a valence of three to a valence of six in the second stage.

During high temperature oxidation in accordance with the invention, carbon is oxidized and eliminated, the degree of elimination of carbon depending upon the time and temperature of roastin or oxidation and the eiilciency of contact of the oxygen of the air with the particles of material undergoing treatment. The degree of elimination of carbon is, in eiiect, a measure of the degree of oxidation of the iron and chromium, as carbon retained in the product is largely or entirely present in unoxidized particles of ferrochromium. In practicing my invention, I may so conduct the oxidation treatments as to eflect complete or substantially complete oxidation of the iron and chromium (with substantially complete elimination of carbon) or I may so control the operation as to retain in the oxidized product some unoxidized ierrochromium. The latter procedure may be desirable at times as a matter of economy, as the rate of oxidation tends to decrease as complete oxidation is approached. At high temperatures, oxidation of metallic iron and metallic chromium of ferrochromium proceeds rapidly and efliciently, whereas, at low temperatures, the metallic iron and metallic chromium of ferrochromium are diflicult to oxidize even under the most favorable conditions with respect to contact with the oxidizing agent. Therefore, it is essential that the high-temperature stage or phase of the process of the invention be controlled to eilect the desired degree of oxidation of iron and chromium from the metallic state. The low temperature stage or phase of the process can not be relied upon to efiect oxidation of residual metallic iron and metallic chromium, as the normally unfavorable oxidizing conditions become even more unfavorable by virtue of dilution resulting from oxidation of the major portion of the metallic iron and metallic chromium in the high-temperature stage or phase. Residual metallic iron and metallic chromium in the product of the high-temperature stage or phase are oxidized very little or not at all in the low-temperaremains unoxidized, I may treat the residual iron oxide-bearing material (after separation of the hexavalent chromium) to eiiect its recovery, and I may retreat the unoxidized ferrochromium thus recovered alone or with additional ferrochromium. Separation of the unoxidized ferrochromium from the residual iron oxide-bearing material may be accomplished readily by gravity concentration methods, as, for example, by means of tabling with water or by means of hindered settling in water. The unoxidized ferrochromium may be recovered also by magnetic separation. The feature of my invention involving the recovery of unoxidized ferrochromium may be employed advantageously in conjunction with processes of the type of that described in my aforementioned patent, employing only low temperature oxidizing treatments.

I have discovered that oxidation of the iron, chromium and carbon of ferrochromium alloys proceeds rapidly and eiiectively when the alloys are heated, in finely divided forms, to temperatures substantially higher than 1200 C., but below the melting points of the alloys. At temperatures below 1000 0., oxidation proceeds more slowly. In roasting ferrochromium containing more than four percent (4%) carbon, for example, at temperatures below 1000 C., oxidized products containing less than about 0.30 percent of carbon (by weight) are difilcult to obtain, and even at temperatures as high as 1200 C., products containing less than 0.20 percent by weight of carbon are difllcult or impossible to produce with many hours of roasting under oxidizing conditions. At temperatures substantially higher than 1200 (3., products containing not more than about 0.10 percent by weight of carbon and containing even as little as a trace of carbon can be produced by roasting under oxidizing conditions for periods of time shorter than two hours. In roasting to effect a high degree of carbon elimination, I prefer to employ temperatures in excess of 1250 C. or even in excess of 1300 C. Operating at temperatures above 1300 C., in the range of 1300 C. to 1350" C., with and without oxidation promoters, I have obtained oxidized products entirely free of carbon (and, consequently, entirely free of metal),- products in which the presence of carbon could not be detected by the usual analytical methods.

In practicing my invention, the ierrochromium may be subjected to the oxidizing treatment alone or in the presence of one orv more additional agents such as lime or soda ash which serve as oxidation promoters and which react with the oxides resulting from oxidation oi the iron and chromium. Additional agents such as lime and soda. ash may be employed in amounts sufilcient to combine with all of the hexavalent chromium produced ultimately, to form chromates, or, such agents may be employed in amounts insuflicient to form chromates with all oi. the hexavalent chromium produced ultimately. When the ferrochromium is subjected to the oxidizing treatment alone in the first instance, the product of such oxidizing treatment may be subjected to a further oxidizing treatment in the presence of one or more agents of the aforementioned type to accomplish further oxidation, to bind the oxides chemically, or to accomplishboth of these objectives. Also, when the ferrochromium is subjected to the oxidizing treatment in the presence of agents such as lime and soda ash in amounts insufiicient to form chromate with all of the hexavalent chromium, the product of such oxidiz- 4 ing treatment may be subjected to a further oxidizing treatment in the presence of an additional amounts of such agents to provide suiiicient to form chromate with all of the hexavalent chromium.

In a preferred process of the invention, 1 subject ferrochromium to the high-temperature oxidizing treatment alone, or in the presence of lime to aid in preventing sintering or fusion. Oxidation of the metallic chromium at high temperatures proceeds satisfactorily without the aid of an oxidation promoter. On the other hand, oxidation promoters like soda ash are definite aids in converting trivalent chromium to the hexavalent condition in the low-temperature stage or phase of the process when oxidation is carried out in the presence of lime to form calcium chromate.

The process of the invention may be employed to form an oxidized product in which the chromium is present largely or substantially entirely as a single chromate compound, or, it may be employed to form an oxidized product containing two or more chromate compounds in any desired proportions. Thus, for example, the process may be controlled to form an oxidized product in which the chromium is present substantially entirely in the form of a chromate of an alkaline earth metal such as calcium, substantially entirely in the form of a chromate of an alkali metal such as sodium, or partly in the form of an alkaline earth metal chromate and partly in the form of an alkali metal chromate. Oxidized products containing the chromium in the form of about ten to twenty, twenty to thirty, thirty to forty, and forty to fifty percent sodium chromate and ninety to eighty, eighty to seventy, seventy to sixty and sixty to fifty percent calcium chromate respectively are very desirable. The presence of such proportions of sodium and calcium chromates simplifies the matter of separating the chromates from the accompanying materials in the oxidized products and mixtures of sodium and calcium chromates are very desirable reagents for use in the production of chromium alloys.

A complete preferred oxidizing treatment of the invention for the production of a product containing chromium largely in the form of calcium chromate may include high temperature oxidation of ferrochromium alone or in the presence of lime to convert the chromium to the trivalent condition, and low-temperature treatment of the trivalent chromium-bearing product in the presence of soda ash (about one to live (1 to 5) percent of the charge by weight) and lime, the lime and soda ash being present in amount sufficient to form chromate with all of the hexavalent chromium produced. The lime may be added in whole or in part either in the high-temperature stage or in the low-temperature stage.

The production of sodium chromate and mixtures of sodium and calcium chromates may be carried out similarly with suitable adjustment in the proportions of soda ash or lime and soda ash employed.

The low-temperature stage may be a combined cooling and oxidizing stage. After completion of the operation resulting in the conversion of chromium to the trivalent condition, the oxidized product is at a temperature higher than that at which conversion of the trivalent chromium to the hexavalent condition will take place, By simply cooling the high-temperature product with access of air and with suitable contact oi lime and an oxidation promoter like soda ash, the trivalent chromium may be converted to the hexavalent condition with the resulting production or chromates of calcium and sodium,

In producing calcium chromate, I prefer to employ. in the charge undergoing treatment for oxidation to chromate, lime in an amount about ten percent in excess of that required to provide two molecules of calcium oxide (CaO) for each molecule of ferric oxide (F8203) produced by oxidation of the iron of the ierrochromium, and two molecules of calcium oxide for each molecule of chromic oxide (CraOa) produced by oxidation of the chromium to the trivalent condition. As hereinbefore pointed out, all or any portion of the lime employed may be added to the oxidation charge at any stage of the oxidizing operation.

The oxidizing treatments of the invention may be carried out in any type of equipment suitable for use in oxidizing or roasting materials in the solid state. Rotary kilns of the type of cement kilns may be employed advantageously. Such apparatus permits efiective heating (also effective cooling if the heat source is eliminated) and eflective contact of the components or the charge with one another and with oxidizing gases.

A grinding operation ma be interposed between the high-temperature stage and the lowtemperature stage. Grinding is not necessary if care is used to prevent fusion or sintering in the high-temperature stage. Grinding may be desirable, however, to efiect thorough mixing of the oxidized product with agents such as lime or soda ash which may be added after completion of the high-temperature stage,

Roasting or oxidizing of high carbon ferrochromium in the solid state to efi'ect a high degree of carbon elimination with substantially complete oxidation of the iron and chromium requires preliminary fine grinding of the ferrochromium. I have found products in which the major portion of the particles are minus 100- mesh to be most suitable for effective removal of carbon and oxidation of iron and chromium by roasting. The roasting of the finely divided particles results in the production of particles of the resulting oxidized compounds of the same general order of size as the particles of ferrochromium treated. These particles appear to consist of agglomerates of even smaller particles formed as a result of oxidation of the individual atoms of the components of the ferrochromium. Roasting of the finely divided ferrochromium tends to produce separated particles approaching the molecular sizes of the compounds formed, and, while the achievement of this objective is impossible, the character of the smaller particles produced, as indicated by the ease with which the particles may be disrupted, indicates that the tendency is not arrested entirely. The roasted product as discharged or withdrawn from the roasting equipment will not necessarily be in the form of a powder as fine as the ferrochromium powder employed in its production. Usually, it will contain a large proportion of ag lomerates of particles of the size of and smaller than the particles of ferrochromium employed in its production. The particles comprising the agglomerates are relatively loosely bound together and they may be separated readily by grinding or even merely by tumbling as in a rotary kiln. Grinding or tumbling of the roasted product breaks up the agglomerates readily and effectively and results such as lime and soda ash which are employed for the ultimate production of chromates should be mixed intimately with the trivalent chromium-bearing product. Such agents preferably are employed in the form of particles minus mesh in size. Intimate mixing may be effected advantageously by grinding any lime and soda ash employed in the first stage with the ferrochromium and by grinding such agents with the oxidized product when they are added after oxidation of the ferrochromium. Simple grinding to produce a product in which ninety percent of the particles are minus IOU-mesh results in conversion of more than fifty percent of the oxidized product to an impalpable powder, a powder in which more than fifty percent of the material (by weight) consists of particles small enough to pass a ZOO-mesh screen. The extremely finely divided particles readily coat and tenaciously adhere to particles of lime and soda ash with which they may be mixed and thus provide the intimate contact required for rapid and thorough oxidation and conversion of the chromium to chromate.

In practicing my invention, advantage may be taken of the effect of roasting in facilitating re-grinding by subjecting the ferrochromium to a preliminary roasting operation in a relatively coarse state of division to accomplish fractional oxidation of chromium to the trivalent condition and subsequently re-grinding and re-roasting one or more times, if necessary, to accomplish the desired degree of oxidation.v For example, the ferrochromium may be ground in the first instance to form a product, the major portion of which consists of particles not substantially smaller than required to pass through a 65-mesh or equivalent screen (Tyler series). After roasting of this relatively coarse product to accomplish eflicient oxidation in view of its relatively coarse nature, the roasted product may be subjected to a grinding operation to form a product, the major portion of which consists of particles small enough to pass a IOO-mesh screen. The resulting more finely divided material may be roasted to effect further oxidation.

Preliminary roasting (followed by ire-grindin and re-roasting) may be carried out with ferrochromium ground initially to any desired particle size. Double or multiple roasting to convert the chromium to the trivalent condition (preliminary roasting followed by re-grinding and re-roasting) may be advantageous for several reasons. For example, the grinding of high carbon ferrochromium to produce a product of which a large proportion consists of particles small enough to pass a IOU-mesh screen is a relatively simple matter. Therefore, the problem of securing an ultimate oxidized product comprising particles of the most desirable small sizes can be simplified by combining the advantages which can be derived as a result of the grinding characteristics of high carbon ferrochromium with advantages which can be derived as a result of the grinding characteristics of the oxidized product.

Multiple roasting to convert the chromium 'to the trivalent condition may be carried out with or without the presence of oxidation promoters in any stage, or, when any stage of roasting is carried out in the presence of one or more of such agents, additional amounts may be added to the product of that stage of roasting before 7 subjecting it to the next stage of roasting. Thus, for example, I may first roast.finely divided ferrochromium at a relatively high temperature above 1000 C. in the presence of suflicient lime to form calcium chromite with all of the chromium in the ferrochromium and subsequently roast the product obtained by such roasting treatment at a temperature below 1000' C. and above about 750 C. in the presence of suillcient additional lime and soda ash to provide a total amount of calcium oxide and sodium oxide to combine with all of the chromic oxide remaining unchanged to form chromates of calcium and sodium.

Roasting operations may be facilitated by selection of the ferrochromium to be treated. Highcarbon ferrochromium products grind more read- Hit and can .be converted to desirably small particles more easily than low-carbon ferrochromium. Thus, for example, ferrochromium containing 8 to 10 percent or more carbon can be comminuted to the form of a powder comprising very small particles quite easily; ferrochromium containing 6 to 8 percent carbon is more diilicultly reducible to particles of desirably small sizes; and ferrochromium containing less than about 6 percent carbon can be reduced or ground to particles desirably small in size only with considerable difllculty relatively to the diillculties encountered in 'finely dividing ferrochromium products containing more than 6 percent carbon.

The grinding of relatively low-carbon ferrochromium is facilitated if the ferrochromium also contains silicon. Thus, for example, ferrochromium containing about 4 to 6 percent carbon and amounts of silicon up to about 3 percent can be ground quite readily.

In practicing my invention, I prefer to employ ferrochromium as nearly saturated as possible with carbon, or, alternatively, ferrochromium containing smaller amounts of carbon, but containing, also, sufllcient silicon to compensate, in its influence upon grinding characteristics, for the carbon deficiency. When I produce the ferrochromium forroasting by reduction treatments of chromium-bearing materials, I prefer to employ suificient carbon to incorporate in the resulting ferrochromium as much carbon as possible, and, if the conditions of operations are not such as to permit the production of products of the higher carbon content, I operate the reductlon process under conditions such as to reduce silicon from silica contained in the charge and form a product containing, preferably, about 1 to 3 percent of silicon. Larger quantities of silicon improve the grinding characteristics of the ferrochromium produced, but they are objectionable because they increase the bulk of the oxidized products.

The following example illustrates a process of my invention: 7

One hundred parts of ferrochromium metal analyzing 61.8% Cr, 8.68% C, 1.5% Si and 28.1% Fe when roasted with 5 parts of CaO at 1350 C. gave 138.5 parts of an oxidized product analyzing as follows:

Per cent CraOa 65.0' F8203 28.9 CaO 3.6 SiOz 2.3

This product, containing 138.5 parts of substantially completely oxidized material was mixed with 99.0 parts of lime (CaO). The resulting mixture, including the 5 parts of lime added to the term-chromium, contained 104 parts of lime (CaO) which is sumcient excess lime for high conversion to calcium chromate. 3.2 parts of soda ash was also added and the whole mix ground in a ball mill to a powder made up chiefly of particles under loo-mesh and a large portion under 200-mesh. The resulting finely divided product was roasted for one-halt hour with access of air at temperatures in the range, 750 C. to 1000 C., with conversion of about ninety-six percent (96%) of the chromium to calcium chromate. The calcium. chromate-bearing product was treated by known methods to recover various products consisting essentially of (1) calcium chromate, (2) calcium bichromate, (3) sodium chromate, (4) sodium dichromate. (5) a mixture of sodium bichromate and calcium bichromate, and (6) chromic acid anhydride.

In the example given above, the product subjected to the low-temperature oxidizing treatment was extremely finely ground. Such fine grinding or comminuting is effective in accelerating oxidation. Coarser products may be oxidized eilectively, but longer periods of treatment are required to achieve substantially complete conversion to chromate.

Chromate compounds may be recovered from the chromate-bearing oxidized products of the invention in any suitable manner. when the oxidize products contain chromium largely or substantially in the form of calcium chromate. the calcium chromate may be dissolved in water or water and acid by known means to give a solution of calcium bichromate which is highly soluble. The acid used may be sulphuric acid. hydrochloric acid, nitric, carbonic acid or chromic acid. In leaching, calcium chromate is relatively highly soluble in the cold and less soluble in the hot. However, calcium bichromate is highly soluble and it is usually preferred to use sufficient acid to give a solution of bichromate of calcium. If the leaching is carefully done and the bichromate stage not exceeded, the iron oxide remains insoluble. After filtration a strong solution of calcium bichromate is obtained from which the calcium bichromate may be crystallized or calcium chromate precipitated by the addition of lime. Any sodium chromate may be precipitated by adding calcium chloride and adjusting the alkalinity with sodium hydroxide.

For treating oxidized products in which the chromium is present largely as calcium chromate. I prefer to employ carbonic acid in an aqueous leaching solution, under about two to three atmospheres pressure to obtain a solution of calcium bichromate, which solution will contain. also, some sodium bichromate if soda ash is used in the roasting or oxidizing treatment. dioxide for producing the carbonic acid may be obtained readily and cheaply from ordinary combustion gases such as stack gases.

I claim:

1. The method of producing hexavalent chromium which comprises maintaining ferrochromium in finely divided form in intimate contact with an oxygen-containing gas at a temperature above 1250 C. but below the fusing point of the ferrochromium until substantially all carbon present is eliminated and chromium contained in the ferrochromium is converted to the trivalent condition without substantial conversion of such chromium to the hexavalent condition, maintaining the trivalent chromium in intimate contact with an oxygen-containing gas at a temperature below 1000' C. in the presence Carbon of a compound from the class consisting of lime and basic alkali-metal compounds until it is contained in the ferrochromium is converted to the trivalent condition, without substantial conversion of such chromium to the hexavalent condition, grinding the product of said heating treatment to form finely divided material, maintaining the trivalent chromium in the finely divided material in intimate contact with an oxygen-containing gas at a temperature below 1000 C. in the presence of a compound from the class consisting of lime and basic alkali-metal compounds until it is converted to the hexavalent condition, and recovering a hexavalent chromium from the resulting chromium-bearing product.

3. The method of producing hexavalent chromium which comprises maintaining ferrochromium in finely divided form in intimate contact with an oxygen-containing gas at a temperature above 1000 C. but below the fusing point of the ferrochromium to eliminate substantially all carbon present and to form agglomerates containing the chromium of the ferrochromium in the trivalent condition without substantial conversion of such chromium to the hexavalent condition, breaking down the agglomerates to form finely divided material, maintaining the trivalent chromium of the finely divided material in intimate contact with an oxygen-containing gas at a temperature below 1000 C. in the presence of a compound from the class consisting of lime and basic alkali-metal compounds until it is converted to the hexavalent condition, and recovering a hexavalent chromium compound from the resulting chromium-bearing product.

4. The method of producing hexavalent chromium which comprises maintaining ferrochromium in finely divided form in intimate contact with an oxygen-containing gas at a temperature above 1000 C. but below the fusing point of the ferrochromium until substantially all carbon present is eliminated and chromium contained in the ferrochromium is converted to the trivalent condition without substantial conversion of such chromium to the hexavalent condition, adding a compound from the class consisting of lime and soda ash to the product of said heating treatment, maintaining the resulting mixture in intimate contact with an oxygen containing gas at a temperature below 1000 C, until the trivalent chromium is converted to the hexavalent condition, and recovering a hexavalent chromium compound from the resulting chromium-bearing product.

5. The method of producing hexavalent chromium which comprises maintaining ferrochromium in finely divided form in intimate contact with an oxygen-containing gas at a temperature above 1000 C. but below the fusing point of the ferrochromium until substantially all carbon present is eliminated and chromium contained in the ferrochromium is converted to the trivalent condition without substantial conversion of such chromium to the hexavalent condition, adding a compound from the class consisting of lime and soda ash to the product of said heating treatment, grinding the resulting mixture to finely divided form, maintaining the finely divided material in intimate contact with an oxygen-containing gas at a temperature below 1000 C. until the trivalent chromium thereof is converted to the hexavalent condition, and recovering a hexavalent chromium compound from the resulting chromium-bearing product.

6. The method of producing hexavalent chromium which comprises maintaining ferrochromium in finely divided form in intimate contact with an oxygen-containing gas at a temperature above 1000 C. but below the fusing point of the ferrochromium until substantially all carbon present has been eliminated and the chromium contained in the ferrochromium is converted to the trivalent condition, without substantial conversion of such chromium to the hexavalent condition, maintaining the trivalent chromium in intimate contact with an oxygen-containing gas at a temperature below 1000 C., in the presence of an amount of lime about 10% in excess of that required to provide two molecules of calcium oxide for each molecule of ferric oxide produced by oxidation of the iron of the ferrochromium and two molecules of calcium oxide for each molecule of chromic oxide produced by oxidation of the chromium to the trivalent condition, until it is converted to the hexavalent condition, and recovering the hexavalent chromium compound from the resulting chromium-bearing product.

7. The method of producing hexavalent chromium which comprises maintaining ferrochromium in finely divided form, in the presence of an amount of lime sufficient to form calcium chromite with all the chromium of the ferrochromium. in contact with an oxygen-containing gas at a temperature above 1200 C. but below the fusing point of the ferrochromium until substantially all chromium contained in the ferrochromium has been converted to calcium chromite without substantial conversion of such calcium chromite to calcium chromate, maintaining the calcium chromite in intimate contact with an oxygen-containing gas at a temperature below 1000 C. in the presence of a compound from the class consisting of lime and basic alkali-metal compounds until the chromium thereof is converted to the hexavalent condition, and recovering a hexavalent chromium compound from the resulting chromium-bearing product.

8. The method of producing hexavalent chromium which comprises maintaining ferrochromium containing from about 8% to about 10% of carbon in finely divided form in intimate contact with an oxygen-containing gas at a temperature above 1000 C. but below the fusing point of the ferrochromium until substantially all carbon present is eliminated and chromium contained in the ferrochromium is converted to the trivalent condition without substantial conversion of such chromium to the hexavalent condition, adding a compound from the class consisting of lime and soda ash to the product of said heating treatment, maintaining the resulting mixture in intimate contact with an oxygen-containing gas at a temperature below 1000 C. until the trivalent chromium is converted to the hexavalent condition, and recovering a hexavalent chromium compound from the resulting chromium-bearing product.

9. The method of producing hexavalent chromium which comprises maintaining ferrochromium containing from about 4% to about 6% carbon and silicon not exceeding about 3% in finely divided form in intimate contact with an oxygen-containing gas at a temperature above 1000" C. but below the fusing point of the ferrochromium until substantially all carbon present is eliminated and chromium contained in the ferrochromium is converted to the trivalent condition without substantial conversion of such chromium to the hexavalent condition, adding a compound from the class consisting of lime and soda ash to the product of said heating treatment, grinding the resulting mixture to finely divided form, maintaining the finely divided material in intimate contact with an oxygencontaining gas at'a temperature below 1000 C. until the trivalent chromium thereof is converted to the hexavalent condition, and recovering a hexavalent chromium compound from the resulting chromium-bearing product.

10. The method of producing hexavalent chromium which comprises maintaining ferrochromium in finely divided form in intimate contact with an oxygen-containing gas and in the presence of a compound from the class consisting of 12 lime and basic alkali-metal compounds at a temperature above 1200 C. but below the main: point of the ferrochromium until the major Portion of the carbon has been eliminated and the iron and chromium of the ferrochromium have been oxidized, with the chromium being oxidized to the trivalent condition, but without substantial conversion of such chromium to the hexavalent condition, adding a compound from the class consisting of lime and soda ash to the product of said heating treatment, heating the resulting mixture while in intimate contact with an oxygen-containing gas at a temperature below 1000' C. until the trivalent chromium thereof is oxidized to the hexavalent condition, separating the hexavalent chromium from the oxidized iron, concentrating the residual oxidized iron-bearing material to separate and recover unoxidiaed ferrochromium contained therein and returning the unoxidized ferrochromium thus separated and recovered to the high temperature oxidation stage of the process.

MARVIN J. UDY. 

